Alumina reduction cells, in which aluminum is produced electrochemically from alumina, consume tremendous amounts of electricity and operate at very high temperatures, typically 960 degrees C. It is difficult to observe and measure the various physical and chemical states within the cell due to the high temperature the cell being enclosed. Measuring the electrical currents into and out of the cell is one of the few measurable parameters. It is therefore important to monitor the current distribution in the cells to gain more understanding of cell phenomena which will lead to improvements in cell efficiencies and reduction in cell instabilities.
Alumina reduction cells operate with direct, as opposed to alternating currents. The cells have one or more anodes distributing current to the cell, one or more cathodes collecting current from the cell and an electrolyte containing the dissolved alumina. Production facilities contain a number of electrolytic cells electrically connected in series. The anodes and cathodes typically have multiple conductors connected to busses to carry the current to the adjacent cell.
The current carried by each conductor in a cell varies due to physical and electrochemical reasons. Physical reasons include the resistance of the connection between the conductor and the buss, resistance variations depending or the conductor's material and its quality, etc. Electro-chemical reasons include the chemical composition of the electrolyte, depth of the electrolyte between the anode and cathode, etc. Beneath the electrolyte is the molten aluminum product. As the aluminum is produced, the electrolyte composition changes, thereby varying its resistance. The size of industrial electrolytic cells result in non-homogeneous electrolyte composition resulting in variations in the current from conductor to conductor.
The magnitudes of the currents in an industrial cell line create significant ambient magnetic effects, large enough to create movements and instability in the liquid metal bath and electrolyte. These movements will change the depth of electrolyte between the anode and cathode and, as described above, vary the currents in the anodes and cathodes. This results in variations in currents in the conductors connected to the anodes and cathodes.
Various attempts have been made to determine the current distribution in the alumina reduction cell. This has been done by measurement of the direct voltage between two points on the anode, and is typically done using "voltage taps". See for example U.S. Pat. No. 4,786,379 issued to Reynolds Metal Company on Nov. 22, 1988. Voltage taps measure the voltage drop at a fixed distance on the conductor in order to determine the current. This existing method has problems with accuracy and reliability. Measurement of voltage differential is problematical due to the small potential differences between the two points of contact and resistance variations due to the temperature of the conductor. As well, they are significantly influenced by the contact resistance between the probe and the conductor which can vary due to such things as the amount of oxidation and deterioration in the contact. The environment in which the conductors operate is detrimental to maintaining a clean contact.
Other problems with existing methods are: 1) safety concerns with equipment electrically connected to uninsulated conductors at high potential; 2) anodes must be changed periodically which may require that sensors encircling the conductor be removed; 3) reliability of electrical and electronic components in the adverse environment in the immediate vicinity of electrolytic cells; 4) induction from large, ambient magnetic fields into control cabling causing distortion in the signals; 5) cell currents are very dynamic and necessitate snapshots of current density for all conductors within a very short period; 6) unreliability of electronic equipment and wiring within the vicinity of large ambient magnetic fields.